System for dispensing carbon dioxide

ABSTRACT

A system for dispensing carbon dioxide, particularly a slurry of particulate solid carbon dioxide dispersed in liquid carbon dioxide, so as to produce solid carbon dioxide in a form suitable for use as a refrigerant. The carbon dioxide is expanded from a source at a pressure above 60 p.s.i.g. to a zone at atmospheric pressure (0 p.s.i.g.) through a positive displacement device which transports a discrete, isolated charge of carbon dioxide from the high pressure source to the atmospheric zone without significant pressure drop. The head pressure on the device, the quantity of liquid carbon dioxide transported, slippage in the device, etc., are controlled to provide for reliable dispensing of slurry, e.g., without plugging of the system.

United States Patent .[191

Muska [111 3,817,045 [451 June 18, 1974 SYSTEM FOR DISPENSING CARBONDIOXIDE [75] Inventor: Allen V. Muska, Berkeley Heights,

[73] Assignee: Airco, Inc., Montvale, NJ.

[22] Filed: Oct. 3, 1972 [21] App]. No.: 294,734

[52] US. Cl. 62/10, 62/35 [51] Int. Cl F25j 1/00 [58] Field of Search62/10, 35, 39, 12, 41

[56] References Cited UNITED STATES PATENTS 579,866 3/1897 Elworthy62/38 2,011,550 8/1935 Hasche 62/10 2,011,551 8/1935 Hasche 62/102,084,474 6/1937 Booth r 62/10 3,354,662 11/1967 Daunt i 62/10 3,395,5468/1968 Sherlock 62/10 FOREIGN PATENTS OR APPLICATIONS 298,910 1/1930Great Britain 62/10 Primary Examiner--Norman Yudkoff AssistantExaminerArthur Purcell Attorney, Agent, or Firm-Roger M. Rathbun; H.Hume Mathews; Edmund W. Bopp [5 7] ABSTRACT A system for dispensingcarbon dioxide, particularly a I slurry of particulate solid carbon.dioxide dispersed in liquid carbon dioxide, so as to produce solidcarbon dioxide in a form suitable for use as a refrigerant. The carbondioxide is expanded from a source at a pressure above 60 p.s.i.g. to azone at atmospheric pressure (0 p.s.i.g.) through a positivedisplacement device which transports a discrete, isolated charge ofcarbon dioxide from the high pressure source to the atmospheric zonewithout significant pressure drop. The head pressure on the device, thequantity of liquid carbon dioxide transported, slippage in the device,etc., are controlled to provide for reliable dispensing of slurry, e.g.,without plugging of the system.

10 Claims, 2 Drawing Figures GAS PELLETIZER P E LLETS PATENTEDJUNI 819143 a 1 7; 0 15 SLURRY I4 GAS PELLETIZER PE LLETS 7 l SYSTEM FORDISPENSING CARBON DIOXIDE This invention relates to a system fordispensing carbon dioxide under high pressure and particularly to asystem for dispensing slurries of particulate solid carbon dioxidedispersed in liquid carbon dioxide, to produce solid carbon dioxide foruse in refrigeration or other systems.

Copending U.S. Pat. application Ser. No. 258,962 filed June 2, 1972,incorporated herein by reference, is directed to a novel cryogenicmaterial, more particularly, a pumpable slurry of finely divided,particulate solid carbon dioxide dispersed in liquid carbon dioxide.This slurry is a mixture of solid and liquid carbon dioxide containingup to about 85 percent solids by weight, with the solid being in theform of finely divided, dispersed particles so that the mixture is fluidand pumpable. Generally, the solids are in excess of about 10, or 14percent by weight and it is desired to have as high a concentration ofsolids as possible and still have a pumpable slurry. Solidsconcentrations in excess of 50 percent by weight are preferred. Theslurry is a two .phase mixture that can exist at any pressure andtemperature along the solid-liquid equilibrium curve for carbon dioxide.The lowest pressure on this equilibrium curve is 75 p.s.i.a. (6Op.s.i.g.) and the lowest tempera ture is 70F., which pressure andtemperature occur at the triple point, the only point on the curve whereall three phases of carbon dioxide, i.e., gas, liquid, and solid, canexist simultaneously. The solid particles in the slurry have a maximumdiameter on the order of 3mm. or less and show little, or no tendency toagglomerate on standing.

While this slurry has significantadvantages over liquid carbon dioxideas a refrigerant, there are problems in dispensing the slurry and, atthe same time, recovering the total refrigeration available in theslurry which are not presented in dispensing liquid carbon dioxide.Normally, dispensing of liquid carbon dioxide is performed in a deviceknown as a snow horn which typically consists of a thin plate having anumber of tiny orifices (e.g., 4 or 5) and a horn to which the plate isat tached. With a completely liquid system, it is necessary only torelease pressure on the liquid in order to recover the refrigeration.

' Upon such a reduction of pressure, as the triple point is approached,the system contains liquid and gas, and, as the triple point is passed,it contains gas and solid. Therefore, liquid, typically availablecommercially at 300 p.s.i.g., is dispensed by allowing it to expandthrough the orifices of the snow horn and form gas and solid. The gasserves to keep the orifices clear of solid and the horn serves mainly toslow the gas. In reducing pressure upon a slurry as it is dispensed,however, the existing solid particles remain unchanged (except thattheir temperature decreases) and the liquid is con verted to gas andsolid, thereby increasing the total amount of solids in the system.While U.S. Pat. No. 3,660,985 discloses a system using very highpressures, 250 to 300 p.s.i.g., to handle carbon dioxide slush of low,i.e., less than 50 percent solids content in a snow horn, the existingsolids in the slurry and the increased total solids in the carbondioxide dispensed render conventional snow horns unreliable asdispensing devices for a slurry because such solids frequently causeplugging, particularly at lower pressures. Therefore, in order torecover the total refrigeration from a slurry of solid and liquid carbondioxide, it is necessary to carefully control dispensing of the slurry.

In accordance with this invention an effective and reliable ,means hasbeen designed to dispense carbon dioxide in the form of a slurry ofsolid particles dispersed in liquid having solids concentrations up toabout percent by weight. In accordance with this invention, carbondioxide slurry, at a pressure in excess of 60 p.s.i.g., is fed into apositive displacement device hav ing a dynamic seal (sliding orrotating) that separates the high pressure carbon dioxide source from alow pressure discharge zone and is transported therein to the dischargezone which is at atmospheric pressure (0 p.s.i.g.). While the use of anexpansion engine to make and maintain a slush cryogen such as hydrogenis shown in US. Pat. No. 3,395,546, the use of such engine is to formslush from slush whereas in the present invention the liquid carbondioxide in the slurry is converted to gaseous and solid carbon dioxideso as to provide solid carbon dioxide refrigerant.

Positive displacement devices suitable for use in this invention mustaccept at the entrance port of the device a discrete charge of slurryfrom the high pressure side into an isolation chamber and transport thecharge in the isolation chamber without significant gas and solidformation in the chamber, i.e., with an essentially negligible orinsignificant pressure drop, to the low pressure atmospheric dischargeor exit port of the device.

It is important that the charge be physically isolated from at least oneof the entrance and exit ports at all times so there is no continuouspath through the device from the high pressure side to the low pressureside. Also, the device must discharge on the low pressure side with aminimum of restriction and each isolation chamber in the device mustdischarge substantially completely to avoid the return of solid to thehigh pressure side in the isolation chamber with attendant buildup ofsolid in the chamber. The gas generated in the device is oftensufficient to remove the solids from the isolation chamber, but it isdesirable to also design the geometry, size and shape, of the chambersand exit port of the device to allow for easy removal of the solids.Removal of the solids can be further assisted by a downward orientationof the exit port although usually the gas generated and geometry aresufficient. if necessary, mechanical devices such as air jets orplungers operating from within the chamber can be used to clear thechamber. Leakage past the dynamic seals in the device from the high tothe low pressure sides must be low. Heat flow into the dispenser fromthe exterior atmosphere during dispensing is preferably minimized byinsulating the body of the device.

The positive displacement device can be a reciprocating, e.g., piston orplunger device, or a rotary, e.g., gear, double helix screw or slidingvane device, but in all cases, the liquid slurry is expanded through thepositive displacement device under controlled conditions. Unlike thesnow horn which operates at constant enthalpy as the pressure islowered, the device of this invention operates between constant enthalpyand constant entropy. Ideally, the expansion should take place atconstant entropy, however, this is not possible in a practical expansiondevice, because constant entropy expansion corresponds to percentefficiency. If no mechanical work is performed, the engine has 0 percentefficiency. The greater the expansion energy that is converted intomechanical work, the greater is the refrigeration effect in the exitingcarbon dioxide stream. Practical devices will have efficiencies betweenthese two extremes, typically 30 to 90 percent. The effect of thisdifference in operation between the displacement device of thisinvention and the snow horn is to produce more solids from the sameamount of liquid in the displacement device as compared to the snow hornbecause some energy is taken out of the fluid as work. The calculated ortheoretical percent solid carbon dioxide formed during the expansion ofcarbon dioxide from various initial conditions to atmospheric pressureis shown by the following table:

The difference in solid carbon dioxide produced represents a significanteconomic advantage to this invention. Accordingly, while this inventionis primarily di rected to the dispensing of a slurry of liquid and solidcarbon dioxide, it can also be used to advantage to dispense liquidcarbon dioxide and gain additional refrigeration per unit of liquid.

Most positive displacement devices have leakage of the liquid beingtransported between the displacement element, e.g., screw, gear, vane,etc., and the casing which contains the displacement element, sometimescalled lip, which reduces the desired isolation of the charge beingtransported from the high pressure source to the low pressure dischargezone. Such slippage is small and effectively moves in the direction ofthe displacing element because of the pressure differential across theelement. If the pressure on the liquid slipping in the displacementdevice is too close to the triple point, additional solids are producedat the area of slipping due to reduction in pressure below the triplepoint because of slippage. Slippage is a problem in the presentinvention because essentially only liquid carbon dioxide slips and theslippage has a tendency to cause plugging of the device. In a geardevice, for example, it packs the area between the teeth with solidcarbon dioxide and prevents the dev ice from transporting the carbondioxide feed. Also, slippage across the face of the gears allowsformation of solids between the face and the gear casing and preventsthe gears from turnmg.

The amount of leakage, or slip per unit of time for a given displacementdevice is dependent mainly upon the pressure drop across the device andis relatively constant for a given pressure drop regardless of therotational speed of the device. To alleviate the problems caused byslippage, and consequent formation of solids in the device, the head, orinlet pressure of the device and the quantity of liquid transported inthe device are controlled. Leakage (slip) is tolerable up to about 50percent but is preferably below about percent of the flow through thedevice. In general, the head pressure on the device is maintained abovethe triple point pressure, i.e., 60 p.s.i.g. and when a slurry is beingdispensed, a sufficient pressure, generally above p.s.i.g., is used toforce the solids in the slurry through the device. The device isparticularly suited for dispensing carbon dioxide at pressures below 150p.s.i.g. Although the slurry can exist at pressures above the triplepoint of carbon dioxide, there is no advantage to using a pressure abovethat necessary to avoid plugging of the device because the enthalpy ofthe slurry is always that of the triple point and the higher pressureoffers no refrigeration advantage. Furthermore, the use of higherpressures will increase slippage by increasing the pressure drop. Thequantity of slurry transported in the device is maintained sufficientlyhigh to supply enough liquid to compensate for the liquid lost byslippage. Since the amount of material lost by slippage in a givendevice per unit of time is dependent mainly upon the pressure dropacross the device and is relatively constant regardless of therotational speed of the device, slippage will add a relatively constantamount of solid to the liquid of the slurry internally of the device,although the total quantity of slurry, and therefore liquid, transportedper unit of time in the device can be increased by speeding up thedevice. The total quantity of slurry transported is always maintainedsufficiently high that the total amount of liquid passing through thedevice, to which the slippage solids are added, is such that theconcentration of solids in the slurry internally of the device isinsufficient to cause plugging. While this internal concentration ofsolids in the slurry is desirably below about percent by weight, it isobvious that both the desired head pressure and the quantity of slurrydesired will be dependant to a large extent upon the particular devicebeing used, e.g., gear, vane, etc., and will vary with both the type ofdevice as well as the size of the device, but in each instance thespecific limits can be quickly determined within the above criteria.

A positive drive is provided for the device of this invention; however,once the device is started, there is little, or no, need to providepositive drive since the head pressure of the carbon dioxide feed willprovide the energy necessary to drive the device. The positive drive canbe used to advantage to slow the device during dispensing and preventuncontrolled operation which could damage it. Also, the positive driveprovides a means of controlling the amount of carbon dioxide dispensedsince, in a positive displacement device, there is a direct relationbetween the amount of carbon dioxide transported and the rotationalspeed of the device. Accordingly, the device can also function as ametering device.

The rate of production of solid carbon dioxide is also a factor incausing plugging of the device. The discharge opening of the device inall instances must be of sufficient size to accommodate the solids andgas generated during discharge and, preferably, the discharge opening isoriented to discharge the solid generated downward. It is also desirableto remove particles having a size larger than the isolation chambers ofthe dispensing device from the slurry. For this reason a screen isnormally placed in the feed line to the device. Since the maximumparticle size in the slurry produced in accordance with copending US.Pat. application Ser. No. 258,962 is about 1 to 3 millimeters, particlesize is not a significant problem.

This invention will bevmore fully described hereinafter with referenceto the drawings in which:

FIG. 1 is a flow sheet of a commercial system in accordance with thisinvention; and

FIG. 2 is a schematic illustration of the preferred em bodiment of thedispenser of this invention utilizing meshed gears.

Referring to FIG. 1, in the system in accordance with this invention,carbon dioxide, for example, a slurry of carbon dioxide produced inaccordance with copending application Ser. No. 258,962 is fed throughline to dispenser 12. FIG. 1 schematically illustrates a system for theproduction of carbon dioxide pellets, a commercially available form ofcarbon dioxide for use in refrigeration systems such as large freezerlockers. In this system the dispenser 12 produces solid carbon dioxidein the form of snow and carbon dioxide gas. The gas is separated in agas separator 14 and the snow is pelletized in a conventional pelletizerunit 16. Carbon dioxide snow can also be used as a refrigerant, ifdesired.

Dispenser 12 is a positive displacement device in accordance with thisinvention and can be any one of several forms including meshed gears,double helix screw,

sliding vane s, etc., which generally are characterized in that thecarbon dioxide is supplied to one-side of the device at a high pressurein slurry form, and the device transports the carbon dioxide indiscrete, isolated charges at a controlled rate to a zone at essentiallyatmospheric pressure, thereby converting the liquid in the slurry tosolid and gaseous carbon dioxide. As discussed above, for properoperation of the dispenser, it is desirable that slippage in thedispenser, head pressure on the dispenser, rotational speed of thedispenser, etc., be controlled.

FIG. 2 illustrates a preferred dispenser incorporating two meshed gears102 and 104 mounted in casing 103, gear 102 being driven by motor 106.In this dispenser, carbon dioxide is introduced from a source, e.g.,line 10 at point A in the direction of arrow 110 at a pressure in excessof 60 p.s.i.g., and often up to 150 p.s.i.g., and is transported topoint B which is at atmospheric pressure, essentially 0 p.s.i.g. Thecarbon dioxide is transported through the dispenser in isolationchambers, e.g., 114, 116 formed between the teeth of gears.l02 and 104,to point B in the direction of arrows 112 and 113. .The product exitingat point B comprises, for example, approximately 18 percent gas and 82percent solids by weight then a 60 percent slurry is provided at pointA. As discussed above, a small amount of the carbon dioxide will leak,or slip through space 118 between the gear teeth and casing 108 in thedirection of arrow 120, e.g., from area 114 to 116, increasing thesolids concentration in area 114. Slippage also occurs across the faceof gears I02 and 104 at each end. Such slippage is kept to a minimum bycontrol of tolerances in the gears and by control of pressure drop, etc.Screen 122 keeps larger particles from entering casing 108.

A number of experimental runs have been made to dispense a slurry usinga device as schematically shown in FIG. 2. The device used had an inlet54 inch diameter, two gears l i inch diameter, and a discharge openingin theform of a rectangle (having rounded comers) about I /4 inch by 2inches. Slurry entering the device was, in the latter runs, passedthrough a screen having a maximum opening of 0.095 inch; the largestdimension between the gear teeth was aboutOLl inch. The solidsconcentration of the slurry ranged'from 10 to 63 percent and the headpressure on the device was between and 140 psi. g. Before transfer ofthe slurry was effected, the device was cooled by feeding liquid carbondioxide at 80 to 100 p.s.i.g. from a storage tank through the dispenserand forming snow. No plugging occurred during these cooling periodsdemonstrating the suitability of the device for dispensing liquid carbondioxide. As the device operates between constant enthalpy and constantentropy, more snow is produced from the liquid upon dispensing throughthis device than through a conventional snow horn.

Table I contains from three representative runs made with the equipment,and operated as described above.

A careful analysis of 34 runs with the device of FIG. 2 revealed thefollowing significant parameters considered essential to the dispensingof a slurry, particularly a slurry of high solid concentration, e.g., 50percent or greater. Firstly, it is necessary to transport the slurryfrom the high pressure source to the exit at atmospheric pressure inrelatively small discrete charges isolated at all times from one of thehigh pressure source and the exit with essentially negligible pressuredrop during the isolation. Secondly, the slippage in the dispensingdevice must be kept to a minimum. Additionally, the head pressure on thedevice must be over 60 p.s.i.g., preferably over 80 p.s.i.g., and thequantity of slurry per unit of time should be sufficient to accommodateany solids produced by slippage. Since large particles in the slurryfeed could clog the device, it is desirable to place a screen at theentrance of the dispensing device to filter the slurry of such particlesand for the device to have a large exit opening.

It is claimed:

1. A method for continuously dispensing a pumpable carbon dioxide slurrycontaining about 10 to percent by weight solid consisting essentially ofparticles of solid carbon dioxide dispersed in liquid carbon dioxide ata pressure of above 80 p.s.i.g. from a closed container at said pressureto a zone at essentially atmospheric pressure, which method comprisesadmitting a charge of said carbon dioxide slurry in sequence to thechambers in a positive displacement metering device having a pluralityof such chambers by placing said chambers in sequence in directcommunication with said closed container to fill said chambers withcarbon dioxide slurry at the same pressure as the pressure in saidclosed container, isolating the charge in each such chamber from saidclosed container, thereafter placing each such chamber in sequence indirect communica- 7 tion with said atmospheric pressure zone to whichthe carbon dioxide slurry is to be dispensed, thereby allowing thecharge to expand into said zone and cause the liquid in the charge toflash to solid and vapor and simultaneously deliver into said zone thesolid mixed with the vapor, and thereafter admitting to said chambersadditional charges of said carbon dioxide slurry to continuouslydispense said slurry, the quantity of said slurry being transported insaid chambers from the container to the atmospheric pressure zone beingsufficient, in combination with the pressure on the slurry, that thetotal amount of liquid passing through the device is such that theconcentration of solids in the slurry internally of the device isinsufiicient to cause plugging of the device.

2. The method of claim 1 wherein the concentration of solids in theslurry internally of the device is below about 85 percent by weight.

3. The method of claim 2 wherein the pressure drop in said chamberduring the isolation of the charge is negligible.

4. The method of claim 2 wherein the slippage of the device is less thanpercent of the flow through the device.

5. The method of claim 2 wherein the slippage of the device is less than50 percent of the flow through the device.

6. The method of claim 2 wherein the pressure in the closed container isbelow 150 p.s.i.g.

7. The method of claim 6 further including filtering the carbon dioxidefeed to the device to remove solid carbon dioxide having a size largerthan the chamber within the device.

8. The method of claim 6 wherein the solid phase carbon dioxidecomprises 50 to 85 percent by weight of the slurry.

9. A method for continuously dispensing a carbon dioxide slurrycontaining about 50 to 85 percent by weight solid consisting essentiallyof particles of solid carbon dioxide dispersed in liquid carbon dioxideat 'a pressure of about p.s.i.g. from a closed container to anatmospheric pressure zone which method comprises admitting a charge ofsaid carbon dioxide slurry in sequence to each chamber in a positivedisplacement metering device having a plurality of said chambers byplacing said chambers without restriction in sequence in directcommunication with said closed container to fill said chambers withcarbon dioxide slurry at the same pressure as the pressure in saidclosed container, isolating the charge in each said chamber from saidclosed container, and thereafter placing each said chamber in sequencein direct communication with said atmospheric pressure zone to which thecarbon dioxide slurry is to be dispensed without restriction, therebyallowing the charge to expand into said zone and cause the liquid in thecharge to flash to solid and vapor and simultaneously deliver into saidzone the solid mixed with the vapor, the pressure drop in each saidchamber during isolation of the charge being negligible, and thereafteradmitting in sequence to said chamber additional charges of said carbondioxide slurry to continuously dispense said slurry, the quantity ofslurry being transported from the container to the atmospheric pressurezone being sufi'icient that the total amount of liquid passing throughthe device, in combination with the pressure, is such that theconcentration of solids in the slurry internally of the device isinsufficient to cause plugging of the device.

10. The method of claim 9 wherein the pressure in the closed containeris below about l50 p.s.i.g.

2. The method of claim 1 wherein the concentration of solids in theslurry internally of the device is below about 85 percent by weight. 3.The method of claim 2 wherein the pressure drop in said chamber duringthe isolation of the charge is negligible.
 4. The method of claim 2wherein the slippage of the device is less than 10 percent of the flowthrough the device.
 5. The method of claim 2 wherein the slippage of thedevice is less than 50 percent of the flow through the device.
 6. Themethod of claim 2 wherein the pressure in the closed container is below150 p.s.i.g.
 7. The method of claim 6 further including filtering thecarbon dioxide feed to the device to remove solid carbon dioxide havinga size larger than the chamber within the device.
 8. The method of claim6 wherein the solid phase carbon dioxide comprises 50 to 85 percent byweight of the slurry.
 9. A method for continuously dispensing a carbondioxide slurry containing about 50 to 85 percent by weight solidconsisting essentially of particles of solid carbon dioxide diSpersed inliquid carbon dioxide at a pressure of about 80 p.s.i.g. from a closedcontainer to an atmospheric pressure zone which method comprisesadmitting a charge of said carbon dioxide slurry in sequence to eachchamber in a positive displacement metering device having a plurality ofsaid chambers by placing said chambers without restriction in sequencein direct communication with said closed container to fill said chamberswith carbon dioxide slurry at the same pressure as the pressure in saidclosed container, isolating the charge in each said chamber from saidclosed container, and thereafter placing each said chamber in sequencein direct communication with said atmospheric pressure zone to which thecarbon dioxide slurry is to be dispensed without restriction, therebyallowing the charge to expand into said zone and cause the liquid in thecharge to flash to solid and vapor and simultaneously deliver into saidzone the solid mixed with the vapor, the pressure drop in each saidchamber during isolation of the charge being negligible, and thereafteradmitting in sequence to said chamber additional charges of said carbondioxide slurry to continuously dispense said slurry, the quantity ofslurry being transported from the container to the atmospheric pressurezone being sufficient that the total amount of liquid passing throughthe device, in combination with the pressure, is such that theconcentration of solids in the slurry internally of the device isinsufficient to cause plugging of the device.
 10. The method of claim 9wherein the pressure in the closed container is below about 150 p.s.i.g.